1. Field of the Invention
The present invention generally relates to an encoder and, more particularly, to an optical encoder which uses the Talbot's interference principle.
2. Description of the Related Art
FIGS. 1A and 1B show principles of a conventional encoder based on the Talbot's interference principle. A light source generally designated at 1 in FIG. 1A consists of a semiconductor laser. A collimator lens 2 converts light beams emitted from the semiconductor laser 1 into plane waves which are incident on a diffraction grating 3 with a period P. Based on the Talbot's interference principle, the light beams diffracted by the diffraction grating 3 form interference fringes having the same period P as that of the diffraction grating at given intervals immediately after the diffraction grating 3. The interference fringes are shown in FIG. 1B. FIG. 1B illustrates sections D.sub.+1, D.sub.0, D.sub.-1 of +1st order, 0th order and -1st order respectively, diffracted light beams each traveling from the diffraction grating 3. The interference fringes are formed in a region indicated by W in FIG. 1B. An image of the interference fringes is called a Fourier image, and this phenomenon is termed a Talbot's effect. A spacing L between the diffraction grating 3 and a diffraction grating 4, having a distribution of transmittances with the same period as that of the diffraction grating 3, is set such that: L=N.multidot.p.sup.2 /.lambda., where .lambda. is the wavelength of the light source, P is the period of the diffraction grating, and N is a natural number. The diffraction grating 3 moves in a g-direction, whereby dark and bright output signals can be obtained via a photosensor 5 disposed behind the grating 4. Thus, pulse signals are generated by an unillustrated signal processing system. A g-directional displacement quantity of the diffraction grating 3 is detected by counting the pulse signals. The photosensor 5 may be in some cases replaced with two pieces of photodetectors A, B (see FIG. 3) to obtain 2-phase sine wave signals.
However, the following problems are inherent in the optical encoder which uses the Talbot's interference principle in the prior art. FIG. 2 shows the 2-phase signals obtained by the optical encoder described above and rectangular wave signals generated therefrom and assuming two phases A, B. A direction in which the semiconductor laser serving as a light source is installed, i.e., a vertical transverse mode of the beam emitted from the semiconductor laser, coincides with an x-direction (namely, the g-direction) in which the diffraction grating 3 moves. In this case, a waveform in an intensity distribution of the Fourier image formed identically with the grating 4 assumes, as illustrated in, e.g., FIG. 2, a more rounded shape on a dark-side m than on a bright-side M. When setting a comparator level (slice level) C for generating square waves taking HIGH and LOW levels at equal intervals from signals taking this configuration, it follows that a setting position is not centered in a signal amplitude but shifts to the dark-side m. Referring to, e.g., FIG. 2, when the comparator level is set at an amplitude center C.sub.O of the signal, a HIGH-to-LOW ratio, i.e., a duty, of the square wave becomes such that HIGH&lt;LOW. A duty ratio cannot therefore be set to 1:1. Further, when employing 2-phase square waves, a HIGH-to-HIGH interval or a HIGH-to-LOW interval of these 2-phase signals is narrowed; or, in other words, a b or d interval is narrower than an a or c interval with respect to phase differences a, b, c, d between a HIGH and a LOW of the pulse. This is conducive to a defect wherein an allowance is lost in an attempt to perform multi-splitting of the signal, i.e., interpolation processing.
Moreover, if the comparator level setting position deviates from the center (comparator level C) of the signal amplitude, there exists a possibility in which the rectangular pulses may not be generated if the amplitude fluctuates due to this deviation and amplitude alternation. Namely, it may happen that the dark-side signal having a smaller amplitude with respect to the comparison level C does not exceed the comparator level because of the fluctuation in amplitude.
Such a distortion in waveform is produced due to the following causes. A Fourier image is produced due to an interference between the +1st order diffracted light and the 0th order diffracted light or between the -1st order diffracted light and the 0th order diffracted light. It is therefore necessary to increase a region to the greatest possible degree where the +1st or -1st order diffracted light is superposed on the 0th order diffracted light on the diffraction grating 4. Turning to FIG. 1B, however, a superposed region of sections D.sub.+1, D.sub.-1 of the .+-.1st order diffracted light beams is large. Formed in this region are fringes having an interval which is half as small as an interference fringe interval of the Fourier image. It follows that these fringes exert adverse influences, such as noise on a detection signal.
The following problems also arise. FIG. 3 is an explanatory view showing a layout of an optical system of the present apparatus. The collimator lens 2 is omitted in FIG. 3. According to the prior art apparatus, the auto power control (APC) oriented photodiode chip 6 is, as shown in FIG. 3, attached in the x-direction of the light source 1. An internal reflection light, or the like, traveling toward the surface of this chip 6 is reflected by this surface towards a z-direction. In this case, the light is mixed as a flare in the A-side of photosensor 5, with the result that unnecessary DC components are included in output signals (2-phase signals SA, SB) of the photo detectors A, B as shown in FIG. 4.